Oscar Pistorius has missed the qualifying time needed for him to become the first amputee sprinter to compete individually at an Olympics.
The South African ran 45.52 seconds in the 400m final of the African Athletics Championships, two-tenths short of the required mark.
He is now unlikely to be selected by the Olympic Committee despite running under the 'A' standard in March.
The 25-year-old is still hoping to be selected in the 4x400m relay team.
The South African selectors had required Pistorius to run under 45.3 seconds for a second occasion, and the race in Benin was his last chance of doing this before London 2012.
"My race today felt good and I'm pleased to have won the silver medal at the African Championships," Pistorius said. "I was in good shape to set the time and believe my speed will only increase over the next few weeks.
"I am also extremely excited about the Paralympic Games in London and a big focus for me will be defending my three titles as well as being part of the South African team competing for a world record in the 4x100m relay at the Paralympics."